1. Field of the Invention
The present invention relates to a new and improved method of determining the charging state of a zinc-bromine battery which contains a plurality of series connected bipolar electrodes and circulating aqueous electrolytes, and further relates to a new and improved method of charging such battery.
2. Discussion of the Background and Material Information
During the storage of energy the charging state of the storage battery or accumulator is just as decisive as the capacity of the storage battery. Only when there can be exactly determined the charging state is it also possible to use the entire capacity of the storage battery.
In the most widely used storage batteries or accumulators for electrical energy, namely, the lead-sulfuric acid batteries, there is usually measured the density of the electrolyte, and thus, there is determined the charging state of the battery. This density measurement is accomplished by removing a partial quantity of electrolyte from the battery and then there is performed hydrometry. This technique is unsuitable for the continuous control of the charging state.
From the text book "Blei- und Stahlakkumulatoren", authored by Erich Witte, 3rd. Edition, Otto Krausskopf publishers, Mainz, Germany, 1967, pages 61 to 63, there is taught the use of a discharge indicator for lead batteries, wherein there is carried out a combined current and voltage measurement. During the current measurement there is used an adjustable shunt resistance. As disclosed in this publication, the determination of the voltage value in the static or quiescent state is distorted due to the combined current and voltage measurement, and furthermore, as is the capacity determination upon reaching a state of approximately 50% discharge. Therefore, the calibration of the indicator instrument as a percentage of the charging state is of only subordinate value since the displayed measurement data are too unreliable.
The electrical potential or voltage of a galvanic element is determined from the potential which is defined by the electrochemical series. However, if two or more oxidation stages exist within the battery, then it is possible for there to arise hybrid potentials of the different oxidation stages. In the case of the lead-sulfuric acid battery, in the charged state thereof, lead peroxide is present at the positive electrode and lead at the negative electrode. Sulfuric acid serves as the electrolyte. During discharge, the tetravalent ions of the lead peroxide are transformed into bivalent ions, and specifically into lead sulfate. At the same time, the metallic lead at the other electrode is likewise transformed into lead sulfate. The potential or voltage at the positive electrode is now dependent upon the concentration of the tetravalent and the bivalent lead ions, and the potential at the negative electrode only upon the concentration of the bivalent lead ions with excess metallic lead. The concentration of the tetravalent ions drops with increasing discharge, whereas, related to the bivalent lead ions, there is present a saturated lead sulfate solution. If there has been consumed the solid, that is, the lead peroxide separated out at the electrode, then the potential rapidly drops to extremely small values, in other words, in the final phase the total voltage or potential drops during discharge of the battery. For determining the charging state of the battery, especially when it has a higher or increased charging state, for instance, where the battery is charged to 80% or 90%, the potential or voltage thus can not serve as an indication or reading. Furthermore, for setting the actual voltage there is required a great deal of time, so that the voltage measurement provides no real value as concerns the capacity of the battery.
In German Published Patent No. 2,028,117, published Dec. 23, 1970, there is disclosed an apparatus for controlling a charging device apparently for a lead accumulator. During charging the actual accumulator voltage is stored in an analog storage or memory. After a predetermined time interval, for example, one-tenth of a second, the actual accumulator voltage is compared with the previously measured value. If there is not present any increase in voltage or potential, then there is interrupted the connection to the charging device. In order to measure the accumulator voltage the charging current voltage must be less than the actual voltage of the accumulator, in order that the measured voltage value does not correspond to that of the charging current source.
From the commonly assigned European Patent No. 0,149,448, published Jul. 24, 1985, there is known a zinc-bromine battery with circulating electrolytes. The electrochemical processes which occur during discharge are such that, on the one hand, metallic zinc goes into solution from the electrode as bivalent zinc bromide and, on the other hand, molecular bromine is converted to bromine ions. The concentration of the zinc and the bromine is constant since, on the one hand, metallic zinc is present at the electrode and, on the other hand, merely because of the equilibrium between a bromine complex and the aqueous electrolyte the bromine concentration is predetermined. Also, equally for the zinc there are present only two forms, namely the zerovalent and bivalent zinc and equally for bromine the zerovalent and monovalent bromine ions.
The bromine complex is difficult to dissolve in water, whereas the complexing agent is dissolved in aqueous electrolytes. The quantity of the difficult to dissolve bromine complex thus constitutes a direct measuring value of the charging state of a zinc-bromine battery of the aforementioned construction. Since the bromine complex has a greater density than water, it separates out or deposits at the floor of an electrolyte container or receptacle. By determining the filling level or height there can be ascertained the charging state of the battery. Considerable errors arise if there is not taken into account the temperature of the electrolyte, since owing to the increase in volume at increased temperatures, or the decrease in volume at lower temperatures, there must be taken into account an additional magnitude. A further disturbance factor, although not present with stationary batteries but, however, with mobile batteries, as such is the case, for example, when used in motor vehicles, resides in the fact that during measurement of the filling level the container or receptacle should be horizontally positioned. If this position does not exist, then, depending upon the inclination of the electrolyte container and the arrangement of the float serving for the filling level measurement, there can be indicated a too low or a too high charging state. Furthermore, the accuracy of such indicator is dependent upon the configuration of the electrolyte container. The smaller the base surface and the greater the height of the electrolyte container that much more accurate becomes the indication or reading. However, with batteries used in mobile environments, the requirement exists that the batteries should be designed to be as flat as possible, and therefore also the electrolyte container or receptacle, so that such filling level measurements are associated with considerable inaccuracies.